1. Field of the Invention
This invention pertains generally to the field of oven heatable plastic coated paperboard containers and to processes for producing the same.
2. Description of the Prior Art
The most common type of container used for convenience foods which are to be heated within the container are formed of a thin sheet aluminum, or layers, which include aluminum foil. These containers are costly and cannot be used in microwave oven cooking. As a result substantial efforts have been made to produce, and provide, plastic coated paperboard cartons or containers which can withstand oven heating and can be used for microwave oven heating.
Polyethylene is the most often used coating material for paperboard since it has good moisture impermeability and is easily adhered to many types of paperboard. Additionally, polyethylene is relatively low in cost. However, polyethylene and many other types of common plastic coating materials do not have sufficient resistance to melting at high temperatures required for conventional oven heating. Coating polymers which withstand conventional oven heating must also have adequate structural strength and abrasion resistance, as well as being FDA approved for product contact.
Polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) are particularly satisfactory coating materials for oven heatable trays. They both exhibit, or possess, high melting temperatures and good structural strength, and are compatible with, and unaffected by, most food products. PET coated paperboard trays, and, to a lesser extent, PBT coated paperboard trays are subject to warpage after heating due to postheating crystallization and shrinkage of the resins. These trays may be unsuitable for certain applications where the tray receives heat prior to final consumer use (e.g., when the packaged food is partially baked in the paper tray prior to distribution to the consumer). It is extremely difficult to obtain good bonding of polyethylene terephthalate or polybutylene terephthalate to other materials and particularly paperboard. Bonding has been accomplished by utilization of adhesives or primers, applied over the paperboard before a hot melt extrusion of the polymer is applied to the paperboard. Concerns have arisen regarding the use of adhesives or primers, due to their possible migration into the contents of the food package.
As a result of the difficulty of bonding polyethylene terephthalate or polybutylene terephthalate to board, procedures or methods to overcome this problem were brought forth.
A procedure for extrusion coating polyethylene terephthalate onto paperboard without the use of primers is shown in U.S. Pat. No. 3,924,013 to Kane. Kane discloses a preheating of the paperboard prior to it being contacted with the hot melt extrusion. Such a process is inadequate for the purposes of a pressed board which is formed into trays. Since preheating the board reduces its moisture content, the board becomes brittle and tears occur during pressing formation. Deep pressed heatable containers or locking corner folded trays are preferred since they do not require the use of adhesives or heat seals to form the edge walls of the tray. Trays formed by adhesively connecting the sides of the tray together are subject to separation at the very high temperatures of oven heating and the adhesive material may migrate into the food product. Pressing allows formation of smooth radius contoured corners, which provides good heat distribution characteristics during oven heating.
A second attempt to overcome the poor adhesion of polyethylene terephthalate is shown in U.S. Reissue Pat. No. 32,270 to Murray, Jr. This document discusses corona discharge treatment of the paperboard prior to the contact with the hot extrudate of polyethylene terephthalate. The melt curtain is extrusion coated at temperatures between 580.degree. F. and 640.degree. F., whereby the temperature of the extrudate, when the web is contacted, is above 480.degree. F. and the temperature to which it is cooled is about 170.degree. F.
It is an object of the present invention to produce a paperboard laminate which can be used in both a conventional oven and a microwave oven. It is a further object of the present invention to produce a dual ovenable container which has good grease and oil resistance and superior food release properties.
It is a further object of the present invention to produce a dual ovenable container from a paperboard laminate which has a novel laminate structure with a food contact layer having food release properties superior to that of polyethylene terephthalate polyester, or polybutylene terephthalate.
It is a further objective of the present invention to produce a dual ovenable packaging structure which resists warpage even after heating.